1. Field of the Invention
The present invention concerns an apparatus for measuring thermal conductivity used for measuring the thermal conductivity of various materials including thermal insulating and heat reserving materials etc, at a steadystate heat flux.
2. Prior Art
Generally the thermal conductivity of various materials used as thermal insulating and heat reserving materials is not steady but varies according to their temperature. Their thermal conductivity increases with the rise in their temperature, i.e. it becomes easy for them to conduct heat. This means that if the thermal conductivity of a material which is used at temperatures exceeding 1,000.degree. C. is to be determined, then the measurement of its thermal conductivity must be done by actually heating it up to its service temperature.
A conventional method for measurement of thermal conductivity is prescribed in ASTM c-177-84 etc., and for example, it is shown in FIG. 18. This conventional apparatus for measuring thermal conductivity is equipped with a main heater b and an auxiliary heater c arranged respectively in the upper and lower parts of a thermally-insulated enclosure a, which heaters produce a steady downward heat flow in the enclosure a, and with a heat flow meter d placed in the upper part of the auxiliary heater c, which plate is designed for the measurement of heat flow quantity of the said steady heat flow.
Generally a device is used whose heat flow meter d contains a gas flow path of a spiral form through which passes the heat-measuring gas. By letting a specified volume of heat-measuring gas heated up to a specified temperature flow through the flow path, the amount of heat received is calculated from the temperature rise and flow rate of the heat-measuring gas.
In the conventional apparatus for measuring thermal conductivity made up as described above, a thermal equilibrium state is created in the enclosure a by means of the main heater b and the auxiliary heater c by arranging a specimen S whose thermal conductivity is to be measured at the center of the enclosure a and by arranging standard heat transfer plates S.sub.1 and S.sub.2 of known thermal conductivity above and below the specimen. The temperature gradient indicated as line B is thus formed through the specimen S and the standard heat transfer plates S.sub.1 and S.sub.2. The thermal conductivity of the specimen S at a particular temperature is calculated from the temperature difference measured by the thermometers e.sub.1 and e.sub.2 between the upper and lower surfaces of the specimen S in a steady state while maintaining the average inside temperature of the specimen S at the temperature to be measured, and from the amount of heat measured by the heat flow meter d at a steady heat flow, i.e. the amount of heat flow through the specimen S.
Hence, let the heat flow measured through the heat flow meter be Q (Kcal/h), the thermal conductivity of the specimen S be .lambda. (Kcal/m.h.deg), the distance from the surface of the specimen S to the inner portion thereof be .delta. (m), the effective sectioned area of the specimen S be A (m.sup.2) and the upper and lower surface temperatures be .theta..sub.1 and .theta..sub.2 (.degree.C.), then the following expression is obtained: EQU Q=(.lambda./t).multidot.A(.theta..sub.1 -.theta..sub.2)
From this equation thermal conductivity .lambda. can be determined as follows: EQU .lambda.-Q.multidot.t/A(.theta..sub.1 -.theta..sub.2) (1)
The standard heat transfer plates S.sub.1 and S.sub.2 serve not only to maintain the specimen S at a high temperature but also to verify and, if necessary, to correct the measured value by comparing the thermal conductivity determined from such surface temperatures and the above-mentioned amount of heat flow Q with the known thermal conductivity values for S.sub.1 and S.sub.2. The symbol g represents heaters for the compensation of wall temperature, which controls the surface temperature of the enclosure a so that the temperature gradient of the enclosure is consistent with the temperature gradient in the enclosure a in order to avoid the heat transfer between the enclosure a and its inner space thereby preventing the flow from dispersing from the peripheral surface of the enclosure a.
In the apparatus for measuring thermal conductivity as described above it goes without saying that obtaining an adequate accuracy of measurement requires the heat flow Q through the specimen S to be measured accurately. Consequently it is important to see to it that there is no dispersion of heat through the peripheral surface to the enclosure a, i.e. the heat flow occurs only downward and not sideward.
Although for this reason the above-mentioned conventional device is provided with heaters g for the compensation of the wall temperature, these heaters alone can not sufficiently prevent the heat in the specimen S and the standard heat transfer plates S.sub.1 and S.sub.2 from flowing sidewards when a testing temperature of the specimen S becomes higher (especially over 1500.degree. C.). This results in an increase of errors in heat flow measurement.
According to the conventional apparatus, because the temperature is measured by means of thermocouples, the highest measurable temperature is limited to be relatively low.